TIbe  TUntpersits  of  Chicago 


DETERMINATION  OF  WILTING 


A DISSERTATION 

SUBMITTED  TO  THE  FACULTY 
OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 
IN  -CANDIDACY  FOR  THE  DEGREE  OF 
DOCTOR  OF  PHILOSOPHY 

DEPARTMENT  OF  BOTANY 


BY 

ARTHUR  LAURENCE  BAKKE 


Private  Edition,  Distributed  By 
THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 
CHICAGO,  ILLINOIS 


Reprinted  from 

The  Botanical  Gazette,  Vol.  LXVI,  No.  2 
August,  1918 


V I h I II  J 

uin'i  '"vmi  ivni 


i i MFISU 


Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


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https://archive.org/details/determinationofwOObakk 


i Jy  2/  lap 


5 &!/  29 

BnJ. 

nR*6^  ! 
REMOTE  STO 

VOLUME  LX VI  NUMBER  2 


THE 

Botanical  Gazette 

AUGUST  1918 

DETERMINATION  OF  WILTING 

CONTRIBUTIONS  FROM  THE  HULL  BOTANICAL  LABORATORY  24 1 

Arthur  L.  Bakke 
(with  five  figures) 

The  status  of  the  question  of  permanent  wilting  in  plants,  as 
described  by  Briggs  and  Shantz  (5,  6,  7,  8),  Caldwell  (ii), 
Shive  and  Livingston  (37),  and  Alway  (i),  centers  about  the 
determination  made  by  Briggs  and  Shantz  that  a plant  is  re- 
garded as  having  attained  a condition  of  permanent  wilting  when 
it  does  not  recover  its  turgidity  in  a period  of  24  hours  when  sur- 
rounded by  air  saturated  with  water  vapor.  The  method  of 
employing  standardized  hygrometric  paper  (2,  3,  4,  28,  30,  38,  40, 
42)  in  the  measurement  of  the  transpiring  power  in  plants  consists 
in  ascertaining  the  power  of  a leaf  to  give  off  water  and  comparing 
this  with  the  power  represented  by  a saturated  blotting  paper  sur- 
face at  the  same  time.  This  is  then  a measure  in  both  cases  of  the 
resistance  to  the  passage  of  water.  The  conditions  which  affect 
such  measurements  are  internal,  but  these  internal  factors  are 
dependent  upon  external  factors.  It  is  obvious,  therefore,  that 
data  derived  will  be  more  or  less  of  a resultant  complex  of  all  the 
forces  which  have  been  operative  during  the  history  of  the  plant. 

The  method  in  principle  is  the  same  as  has  previously  been 
used  in  investigations  upon  the  foliar  transpiring  power  of  plants. 
In  the  present  studies  filter  paper  circles  (Munktell’s  Swedish 

81 


82 


BOTANICAL  GAZETTE 


[august 


no.  oo — ii  cm.)1  are  impregnated  with  3 per  cent  solution  of  cobalt 
chloride  and  are  later  cut  into  small  squares.  Just  before  using, 
these  squares  are  heated  over  a bicycle  lamp,  or  on  a granite  pie- 
plate  suspended  by  a clamp  over  the  flame  of  an  alcohol  lamp,  until 
they  become  blue.  One  of  these  squares  is  placed  between  the 
jaws  of  a “ transpiration  clip,”  and  as  quickly  as  possible  applied 
to  either  the  upper  or  lower  surface  of  a leaf.  The  time  required 
to  change  the  paper  square  from  blue  to  pink  is  determined  in 
seconds.  The  time  which  it  takes  to  change  a similar  piece  of 
cobalt  paper  from  blue  to  pink  when  placed  over  a moist  blotting 
paper  surface  blanketed  by  a millimeter  of  air  (2,  4,  28,  30,  34,  42) 
is  recorded.  The  water  apparatus  is  the  same  as  used  by  Bakke 
and  Livingston  (4).  Trelease  and  Livingston  (42)  have 
developed  the  relations  of  the  temperature  to  vapor  tension  as 
first  shown  by  Bakke  (2).  These  authors  have  presented  a formula 
whereby  the  time  interval  may  be  ascertained  on  knowing  the 
temperature.  Livingston  and  Shreve  (30)  have  recently  im- 
proved and  modified  this  method.  The  principal  improvement  is 
in  the  adoption  of  permanent  color  standards.  Instead  of  the 
simple  square  of  cobalt  chloride  paper,  a composite  slip  is  em- 
ployed consisting  of  a small  piece  of  the  hygrometric  paper  in 
juxtaposition  with  two  slips  having  permanent  color  standards. 
These  provide  both  an  initial  and  an  end  point  for  the  color  change. 
For  use  in  the  laboratory  they  advocate  and  describe  a simpler 
form  of  standard  water  evaporating  apparatus.  These  modifica- 
tions were  not  used  in  this  study. 

The  possibilities  of  using  the  original  method  of  standardized 
hygrometric  paper  in  determining  the  extent  of  wilting  and  the 
permanent  wilting  point  was  first  suggested  to  me  by  its  author, 
B.  E.  Livingston,  at  the  Desert  Laboratory  of  the  Carnegie  Institu- 
tion during  the  summer  of  1913.  In  1914  the  writer  (3),  working  at 
the  Desert  Laboratory,  performed  a series  of  measurements  upon 
sunflower  plants  lifted  from  the  soil  and  later  brought  into  the 
laboratory  to  wilt.  The  results  of  this  series  of  tests  show  that 

1 Livingston  and  Shreve  in  a more  recent  publication  (Improvements  in  the 
method  for  determining  the  transpiring  power  of  plant  surfaces  by  hygrometric  paper. 
Plant  World  19:  287-309.  1916)  have  recommended  Whatman’s  filter  no.  30  (11  cms.) 
circles  as  being  superior  to  the  Swedish  paper. 


1918] 


BAKKE— WILTING 


83 


wilting  occurs  at  a definite  point  and  that  permanent  wilting 
represents  the  most  intense  wilting  possible,  without  serious 
rupture  of  the  water  columns  of  the  plant.  These  studies  have 
been  amplified  in  the  present  investigation.  The  experimentation 
involved  in  the  present  study  was  performed  in  the  greenhouse 
of  the  University  of  Chicago  during  the  summers  of  1915  and 
1916.  The  large  Russian  variety  of  the  common  sunflower  ( Heli - 
anthus  annuus)  was  used,  the  seed  being  from  W.  W.  Barnard  of 
Chicago.  The  experiments  involving  the  porometer  were  per- 
formed in  the  laboratory  of  Plant  Physiology  of  Iowa  State  College. 
The  plants  were  the  same  variety,  but  seed  was  secured  from  the 
Iowa  Seed  Company  of  Des  Moines,  Iowa. 

Series  of  1915 

METHOD 

The  seeds  used  in  the  tests  for  1915  were  planted  in  sheet  iron 
containers  6X6  inches,  on  June  31.  Germination  was  forced  by 
placing  the  containers  in  a warm  house.  When  the  cotyledons 
had  made  their  appearance,  the  seedlings  were  thinned  out  so  that 
only  3 remained.  The  cultures  were  then  removed  to  a cooler  place, 
where  the  plants  were  allowed  to  grow  until  they  were  approxi- 
mately 6 weeks  old  and  about  40  cm.  high.  The  soil  used  in  this 
series  consisted  of  4 parts  of  compost  and  1 part  sand.  The  water- 
holding capacity  was  calculated  to  be  47  per  cent.  The  plants 
remained  in  the  same  containers  throughout  the  entire  period  of  the 
experiment.  They  were  watered  from  time  to  time  until  the 
morning  of  July  13,  when  they  were  heavily  watered,  and  after 
that  no  more  water  was  added  until  the  morning  of  July  16,  when 
the  plants  were  lightly  watered  and  the  soil  surface  covered  with 
plasticine.  Two  plants  were  used  as  checks  in  testing  out  wilting 
by  the  Briggs  and  Shantz  method. 

The  values  for  the  indices  of  foliar  transpiring  power  were 
obtained  according  to  the  original  Livingston  method;  the  stand- 
ard water  apparatus  was  the  same  as  described  by  Bakke  and 
Livingston.  Throughout  the  series,  cobalt  paper  squares  made 
from  Munktell’s  Swedish  no.  00  filter  paper  were  used.  As 
the  work  was  carried  on  in  the  greenhouse,  the  usual  bicycle  lamp 


84 


BOTANICAL  GAZETTE 


[august 


for  lighting  was  replaced  by  electric  light.  The  cobalt  paper 
squares  were  warmed  upon  a granite  pie-plate,  which  was  adjusted 
by  a clamp  over  an  alcohol  flame,  so  that  the  paper  squares  were 
heated  to  a temperature  sufficient  to  give  them  the  blue  color. 

EXPERIMENTATION 

The  readings  for  the  1915  series,  begun  on  August  16,  were 
usually  made  between  the  10th  and  nth  hours  and  again  between 
the  20th  and  21st  hours.  Two  plants  were  used  for  the  foliar  tran- 
spiring power  tests;  two  additional  plants  were  used  for  the  wilting 
determinations  according  to  the  method  of  Briggs  and  Shantz. 
Evaporation  was  determined  at  the  same  time  by  a standardized 
Livingston  form  of  cylindrical  atmometer.  The  readings  as 
recorded  in  table  I show  the  maximum  foliar  transpiring  power 
as  occurring  about  the  nth  hour,  while  the  minimum  usually  occurs 
after  sunset.  Wherever  possible,  leaves  of  different  ages  were 
used  and  were  numbered  and  tagged  I aXl  I a2,  I a3,  I bz,  I b2,  etc.,  the 
highest  number  representing  the  youngest  leaf.  In  this  way  the 
same  leaf  could  be  used  throughout. 

The  average  results  of  the  foliar  transpiring  power  indices, 
as  represented  graphically  (fig.  1),  show  a general  decline  from 
August  16  to  August  20.  The  maximum  index  reached  on  Au- 
gust 17  possesses  a value  of  0.89.  This  index  is  almost  the 
same  as  the  one  obtained  earlier  by  Bakke  and  Livingston. 
Although  the  plants  were  watered  a little  on  the  day  the  experi- 
ment was  begun,  they  must  have  given  off  considerable  water 
during  the  previous  3 -day  interval.  That  the  soil  moisture  con- 
tent has  an  appreciable  effect  upon  foliar  transpiring  power  has 
been  proven  previously,  and  from  the  nature  of  transpiration  it  is 
self-evident.  The  Helianthus  plants  of  Bakke  and  Livingston 
were  growing  in  a place  where  the  soil  moisture  was  less  than  would 
be  regarded  as  optimum.  In  all  probability  the  two  sets  of  Helian- 
thus plants  were  grown  in  soil  having  practically  the  same  amount 
of  moisture.  The  soil  moisture  content  in  both  series  was  below 
the  amount  necessary  for  the  production  of  the  greatest  growth. 

For  the  first  half  of  the  series  the  highest  transpiring  power 
occurs  during  the  day,  while  the  lowest  transpiring  power  values 


BAKKE—  WILTING 


85 


1918] 

are  at  night.  The  average  day  values  are  accordingly  0.72,  0.92, 
0.74,  0.38,  0.26,  0.19,  0.32  for  one  set  (la);  for  the  other  (16), 
0.61,  0.89,  0.76,  0.30,  0.30,  0.39,  0.42.  The  average  night 
values  for  the  first  series  are  0.29,  0.34,  0.24,  0.19,  0.25,  0.44, 
0.69;  for  the  second  series,  0.31,  0.39,  0.23,0.16,0.50,0.45,0.61. 
The  results  obtained  by  calculating  the  ratio  of  the  respective  day 
and  night  values  are  rather  uniform.  For  August  16  the  average 
ratio  is  2.4;  for  August  17,  2.7;  the  remaining  values  for  I a are 
3.1,  2.0,  1.0,  0.43,  0.46.  The  corresponding  respective  values 
for  series  lb  are  2.0,  2.3,  3.3,  1.9,  0.77,  0.87,  0.61.  For 
the  first  two  days,  August  16  and  17,  the  probable  normal  ratio 
is  between  2 and  3.  On  the  following  day  there  is  a slight  increase, 
and  after  that  there  is  a decrease.  Whether  the  rise  in  the  ratio  on 
the  third  day  presents  a normal  situation  or  not  cannot  at  present 
be  stated;  at  any  rate  the  value  is  not  far  from  3.  The  decrease 
in  foliar  transpiring  power  after  August  19  and  the  resulting 
decrease  in  the  ratio  do  not  show  any  definite  mathematical  relation. 
For  a plant  growing  in  a normal  environment,  a rise  in  evaporation 
will  give  an  increase  in  transpiring  power,  but  on  August  22  there 
is  a high  evaporation,  a low  foliar  transpiring  power,  and  a lower 
day  value  than  night  value.  Such  a status  must  be  looked  upon  as 
abnormal  for  growing  plants.  Beginning  with  August  21  there  is  a 
rapid  ascent. 

Considerable  agreement  is  present  between  the  graphs  in  this 
series  and  the  one  for  Helianthus  (3),  where  the  plants  were 
lifted  from  the  soil.  There  is  a decrease  in  the  foliar  transpiring 
power  to  a point  where  there  is  more  or  less  of  a balance,  and  then 
again  where  there  is  an  increase.  The  time  element  in  the  present 
series  is  extended  over  a longer  period,  and  as  a result  variations 
which  might  be  masked  in.  the  series  of  short  duration  would  be 
present. 

The  rupture  of  the  water  columns  of  the  plants  of  the  1915 
series  is  as  definite  as  that  presented  for  the  plants  lifted  from  the 
soil  in  southern  Arizona.  The  outstanding  feature  of  the  curve  is 
the  very  marked  rise  on  August  20.  Upon  examination  of  the 
rate  of  evaporation,  it  will  be  at  once  evident  that  the  evaporating 
power  of  the  air  was  very  low  throughout.  Two  plants  of  this 


Indices  of  foliar  transpiring  power  for  3 different  leaves  of  two  Helianthus  plants  during  progressive  march  of  wilting 
from  August  16-22,  1915  (maxima  in  bold  faced  type,  minima  in  italics) 


86 


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[august 


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'0  t" 

x>.  CM  CM 

O'  M VO  CM  0 O' 00  VO 

M CM 

CM 

CM  CM 

CO  CM 

M M M 

H M 

HO  CO 

CM  CM  h CM  VO  CM 

CM  CO 

0 0 

0 

0 0 

O O 

000 

O O 

000 

OOOOOO 

0 d 

H O 

H 

O CO  O CM 

O '■O  co  co  O' 

H t^-'O 

O'  -cj-  CM  CM  00 

tJ-OO 

CO  to  fO  fO  fO  fO  ^ 

CM  CM  CM 

M CO  CO'O  •'t' 

CO  COM  rJ-vO  rj- 

co 

0 0 

0 

O O 

O O 

000 

O O 

OOO 

OOOOOO 

O O 

0) 

O 

<D 

<u 

<u 

0) 

bo 

bO 

bD 

bD 

bD 

b€ 

e3 

ci 

aj 

«3 

aj 

a} 

IH 

<D 

<D 

<D 

0 

<u 

<D 

> 

> 

> 

> 

> 

> 

< 

< 

<n 

c 

O 

CM 

cm 

CM 

CM 

4-> 

4-> 

W 

tn 

c/3 

P 

0 

3 

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P 

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od  0 

bo 

0 

bD  O 

P 

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p 

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c 

H 

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M 

H 

* Since  the  experiment  has  been  in  progress  a new  leaf  has  become  of  sufficient  size  so  that  it  was  large  enough  for  the  application  of  the  clip.  For  August  21  then 
the  index  0.72  is  much  higher  than  any  of  the  others.  This  will  naturally  increase  the  average.  In  computing  the  average  for  the  index,  this  last  number  was  not 
used;  o. 77  is  the  average  for  leaves  Ifti  and  I b2. 


88 


BOTANICAL  GAZETTE 


[august 


same  series  were  used  for  the  determination  of  wilting  according 
to  the  method  of  Briggs  and  Shantz.  The  results  are  given 


Fig.  i 


in  table  II;  those  obtained  giving  the  residual  moisture  at  the 
time  of  wilting  agree  rather  closely  for  the  determinations  made 
according  to  the  two  methods.  In  the  method  of  Briggs  and 


TABLE  II 


Method 

i 

2 

Average 

Briggs  and  Shantz 

10.38 

7.84 

9. 11 

Hygrometric  paper 

8.20 

8-34 

8.27 

Shantz  there  is  a greater  variation  than  is  found  to  be  present 
where  the  hygrometric  paper  is  used. 


BAKKE— WILTING 


89 


1918] 

The  breaking  point  occurring  on  August  21  is  not  far  from  the 
normal  minimum  value  of  the  daily  march  of  foliar  transpiring 
power.  From  previous  work  upon  the  march  of  foliar  transpiring 
power,  there  is  more  or  less  of  a definite  maximum  (usually  during 
the  day)  as  well  as  a definite  minimum  (usually  during  the  night). 
It  seems  that,  in  all  probability,  the  minimum  in  the  foliar  tran- 
spiring power  indicates  approximately  the  greatest  resistance  to 
transpirational  water  loss.  If  the  water  content  of  the  soil  coupled 
with  the  evaporating  power  of  the  air  is  of  such  magnitude  as  to 
increase  the  resistance  to  the  passage  of  water,  so  that  the  day 
maximum  has  a value  as  low  or  lower  than  the  diurnal  minimum 
(at  night),  the  plant  is  then  in  a critical  condition;  at  least  this  has 
been  found  to  be  true  for  Helianthus.  For  the  entire  leaf  surface 
the  transpiring  power  ratios  at  night  are  as  follows:  (1)  la,  —0.23, 
0.30,  0.34;  (2)  I b,  —0.30,  0.31.  On  August  20  the  respective 
values  are  o . 24,  o . 26,  o . 27  for  I a2,  and  o . 28  and  0.32  for  I b2.  The 
average  ratio  for  the  first  is  2 . 2 and  the  average  ratio  for  the  test  on 
August  20  is  o . 91.  On  August  21  the  ratio  is  less;  on  the  following 
day  it  is  a little  higher. 

The  entire  situation  as  here  brought  forward  centers  about  the 
amount  of  moisture  present  in  the  soil  during  the  march  of  wilting 
when  the  index  of  transpiring  power  ratio  of  day  to  night  comes  to 
be  represented  by  unity  or  less.  The  duration  of  this  ratio  may  be 
an  important  factor  in  obtaining  data  that  will  give  information 
on  the  relative  drought  resistance  of  plants. 

Series  of  1916 

METHOD 

The  method  of  procedure  in  the  experimentation  for  1916  was 
much  the  same  as  for  the  previous  season.  The  sheet  iron  con- 
tainers were  a little  deeper  (7  inches  instead  of  6).  The  soil  mix- 
ture was  lighter  than  before,  containing  1 part  of  clean  pure  sand 
mixed  with  3 parts  of  rich  garden  soil,  and  the  variety  the  same  as 
before  (Mammoth  Russian).  The  seeds  were  planted  on  June  24, 
and  on  July  1 the  seedlings  were  5 cm.  high  and  were  then  trans- 
planted. Three  plants  were  set  deeply  in  the  soil.  The  cultures 


go 


BOTANICAL  GAZETTE 


[august 


were  then  placed  in  the  greenhouse  and  were  watered  from  time  to 
time.  A Livingston  standard  atmometer  of  the  cylindrical  form 
was  set  up  in  close  proximity  to  measure  evaporation.  Readings 
were  taken  of  the  atmometer  whenever  a reading  was  made  of  the 
transpiring  power.  On  July  19  the  plants  were  thoroughly  watered 
and  were  lightly  watered  again  on  July  20.  On  July  21  the  con- 
tainers were  sealed  over  with  plasticine  preparatory  to  making 
hourly  readings  of  the  foliar  transpiring  power  for  a period  of 
twenty-four  consecutive  hours.  For  the  measurements  upon  the 
index  of  foliar  transpiring  power  the  same  apparatus  as  employed 
before  was  used.  On  the  18th  hour  of  July  22  the  last  reading  was 
made  for  the  daily  march  of  foliar  transpiring  power.  Beginning 
July  24,  readings  were  taken  three  times  during  the  day:  (1)  at 
approximately  10:00  a.m.,  (2)  at  the  14th  or  15th  hour,  and  (3)  at 
some  time  during  the  night.  The  times  chosen  really  represent  the 
three  important  periods  during  the  daily  march,  for  the  first  one 
gives  this  value  at  a time  when  the  transpiring  power  is  near  its 
maximum,  the  second  when  evaporation  is  at  its  maximum,  and 
the  third  when  the  index  of  transpiring  power  has  its  lowest  value. 
The  leaves  were  tagged  as  before  so  that  the  same  leaves  were  used 
throughout. 

The  soil  surface  of  several  additional  plants  was  coated  over 
with  plasticine  to  serve  as  a comparison  or  check  for  the  plants 
used  for  the  determination  of  foliar  transpiring  power.  In  apply- 
ing the  cobalt  paper  squares  from  day  to  day,  it  became  easy  to 
judge  the  condition  of  the  plant.  When  plants  presenting  a 
physical  state  such  as  was  in  evidence  for  leaf  I a2  on  August  3,  and 
for  leaf  I b2  on  August  7,  were  placed  in  a moist  chamber,  they  failed 
to  recover.  It  was  then  deemed  unnecessary  to  test  further.  At 
the  time  of  the  beginning  of  the  experiment  plant  la  was  25  cm. 
high,  while  plant  lb  was  28  cm.  high. 

INDICES  OF  FOLIAR  TRANSPIRING  POWER 

In  using  the  method  of  standardized  hygrometric  paper  for 
the  determination  of  the  indices  of  foliar  transpiring  power,  two 
separate  plants  were  used.  The  method  of  numbering  the  leaves 
was  the  same  as  for  the  1915  series.  From  plant  la  two  leaves 


BAKKE— WILTING 


91 


i9i8] 

were  chosen,  I ax  having  the  dimensions  5X8  cm.,  and  Ia2,  4X6  cm. ; 
from  plants  16  two  leaves  were  chosen,  I blf  7X9  cm.,  and  I62,  3X4 
cm.  Whenever  a new  leaf  became  sufficiently  large  for  the  applica- 
tion of  the  clip,  approximately  3X4  cm.,  it  was  included  with  the 
others.  The  readings  were  begun  on  the  18th  hour  of  July  21 
and  continued  at  hourly  intervals  for  24  hours.  Readings  were 
taken  at  the  same  time  from  a standardized  Livingston  cylindrical 
form  of  atmometer.  The  results  for  plant  I a are  given  in  table  III. 

This  series  shows  that  the  march  of  the  foliar  transpiring 
power  is  the  same  as  has  previously  been  pointed  out  (2,  4,  28,  40), 
in  that  the  maximum  transpiring  power  is  attained  at  a time 
previous  to  the  greatest  evaporation.  The  highest  index  occurs 
usually  at  the  10th  and  nth  hours,  while  the  evaporation  maximum 
occurs  usually  on  the  14th  hour.  On  account  of  the  larger  num- 
ber of  readings  it  is  to  be  expected  that  the  graphical  representation 
(fig.  2)  will  show  less  abruptness  than  has  formerly  been  presented. 

Recalling  that  the  leaf  represented  by  Iax  is  older  than  Ia2,  it 
is  plain  that  the  index  of  foliar  transpiring  power  is  higher  for  the 
younger  leaf  almost  entirely  throughout  the  24-hour  period.  The 
maximum  for  I ax  is  at  the  nth  hour,  when  it  is  0.93.  This  value 
is  again  in  evidence  2 hours  later.  For  I a2  the  highest  value  is  at 
the  10th  hour,  when  the  transpiring  power  value  is  1.00.  This 
same  value  is  again  reached  at  the  12th  hour.  It  is  evident  that 
the  younger  leaf  Ia2  reaches  its  maximum  at  an  earlier  period  than 
the  older  leaf  laz.  This  feature  substantiates  similar  conclusions 
reached  by  Bakke  and  Livingston.  Another  important  feature 
in  connection  with  the  graph  showing  the  march  of  foliar  transpir- 
ing power  is  the  sudden  drop  for  both  leaves.  The  lowest  point 
(0.59  for  Ich  and  0.67  for  I a2)  occurs  on  the  14th  hour.  At  the 
15th  hour  the  index  values  are  respectively  0.91  and  0.93,  while 
the  corresponding  values  at  the  13th  hour  are  0.93  and  0.83. 
Although  the  drop  is  the  feature  in  the  afternoon  readings,  the 
recovery  occurring  at  the  15th  hour  is  always  below  that  of  the  fore- 
noon maximum.  In  the  present  case  there  is  not  much  difference, 
being  o . 91  for  I az ; for  the  younger  leaves  there  is  a greater  variation, 
being  0.93  at  15:00  o’clock  and  1.00  for  the  10:00  and  13:00 
o’clock  readings.  At  the  14th  hour  the  average  reading  for  the 


92 


BOTANICAL  GAZETTE 


[august 


TABLE  III 


Data  for  march  of  indices  of  foliar  transpiring  power  for  Helianthus  plant  i a 
(maxima  in  bold  faced  type,  minima  in  italics) 


Leaf 

Time  of 

Time  of  color 

CHANGE  IN  SECONDS 

Index  of  transpiring  power 

Evaporation 

from 

number 

OBSERVATION 

Lower 

surface 

Upper 

surface 

Lower 

surface 

Upper 

surface 

Entire 

leaf 

standardized 

ATMOMETER.CC. 
PER  HOUR 

Ifli 

July  21 
18:15 

31 

52 

0.77 

0.46 

0.62 

I a2 

18:20 

24 

46 

1 .00 

O.52 

0.76 

Iat 

19: 10 

42 

65 

0.68 

O.44 

0.56 

0.22 

I#2 

19:15 

38 

55 

0-75 

O.52 

0.64 

la, 

20: 15 

46 

59 

0.62 

O.49 

0.56 

015 

I a2 

20:  20 

40 

54 

0-73 

0-54 

0.64 

Ia i 

21:10 

45 

72 

0.64 

0.40 

O.52 

O.  l6 

Ia2 

21 : 10 

40 

50 

0-73 

0.58 

0.66 

Iai 

22: 10 

5i 

77 

0.61 

0.40 

0.51 

0.09 

Ia2 

22:15 

40 

65 

0.78 

0.48 

0.63 

Iai 

23:10 

56 

74 

0-57 

0-43 

0.50 

0.15 

Ia2 

23:10 

45 

69 

0.71 

0.46 

o-59 

Iax 

July  22 

24: 10 

50 

90 

0.66 

0-37 

0.52 

O.  l8 

Ia2 

24:15 

42 

77 

0.79 

0.43 

0.62 

Iai 

1 : 10 

48 

88 

0.71 

0.38 

0-55 

O.  12 

Ia2 

1:15 

42 

74 

0.81 

0.46 

0.64 

Iai 

2: 10 

57 

90 

0.63 

0.40 

0.52 

0-15 

Ia2 

2:15 

49 

85 

0.74 

0.42 

0.58 

Iai 

3:10 

55 

85 

0.65 

0.42 

0-54 

0.15 

Ia2 

3:i5 

50 

75 

0.72 

0.48 

0.60 

Iax 

4: 10 

50 

85 

0.72 

0.42 

0-57 

0.15 

I#2 

4:i5 

46 

78 

0.78 

0.46 

0.62 

Iai 

5:10 

50 

62 

0.72 

0.58 

0.65 

0.15 

Ia2 

5:i5 

45 

57 

0.80 

0.63 

0.72 

Iax 

6: 10 

39 

50 

0.87 

0.68 

0.78 

015 

Ia2 

6:15 

36 

50 

0.94 

0.68 

0.81 

Iai 

7: 10 

36 

42 

0.81 

0.69 

0-75 

015 

Ia2 

1 : 10 

36 

37 

0.81 

0.78 

0.80 

Iar 

8:15 

30 

42 

0.77 

0-55 

0.66 

015 

Ia2 

8:20 

28 

32 

0.82 

0.72 

0.77 

Iar. ...... 

9:05 

24 

24 

0.88 

0.88 

0.88 

0.22 

Ia2 

9:05 

22 

22 

0-95 

0-95 

0-95 

Iax 

10:05 

22 

24 

0.95 

0.88 

0.92 

O.58 

Ia2 

10:05 

21 

21 

1 .00 

1 .00 

1 .00 

Iax 

Ia2 

11:00 

11:00 

21 

20 

20 

20 

0.91 

0-95 

0.95 

0-95 

0.93 

o-95 

Iai . 

12 : 00 

21 

21 

0.86 

0.86 

0.86 

I Oa 

12:00 

18 

18 

1 .00 

1 .00 

1 .00 

Iai 

13:10 

16 

16 

o-93 

0.93 

o-93 

I .O 

I a2 

13:10 

18 

18 

0.83 

0.83 

0.83 

Iax 

14: 10 

17 

i7 

o-59 

o-59 

o-59 

I . 2 

1 02 

14:15 

15 

15 

0.67 

0.67 

0.67 

Iax 

15: 10 

15 

16 

o-93 

0.88 

0.91 

I . 2 

Ia2 

15:15 

15 

i-5 

o-93 

0-93 

0-93 

Iai 

16:15 

17 

18 

0.88 

0.88 

0.88 

1.6 

Ia2 

16:  20 

21 

21 

0.71 

0.71 

0.71 

Iai 

17:00 

19 

19 

0-95 

0-95 

0.95 

i .0 

Ia2 

17:00 

18 

20 

1 .00 

0.90 

o-95 

Iai 

18:00 

25 

35 

0.80 

o.57 

0.69 

1 .0 

Ia2 ....... 

18:05 

20 

25 

1 .00 

0.80 

0.90 

BAKKE— WILTING 


93 


1918] 


older  leaves  (lax)  is  0.59  and  for  the  younger  leaves  is  0.67.  The 
differences  then  in  the  order  given  are  0.33  and  0.34.  The  respec- 
tive values  on  the  15th  hour  are  0.91  and  0.93.  These  give 
recovery  value  differences  of  0.32  and  0.26.  The  drop  in  the 
afternoon  reading  is  not  a new  thing,  either  in  foliar  transpiring 
power  or  in  transpiration.  No  doubt  this  great  resistance  to  the 


passage  of  water  is  a condition  of  incipient  drying.  It  may  be  that 
at  this  period,  usually  present  at  about  the  time  of  greatest  evapora- 
tion, there  is  a lack  of  water,  not  only  in  the  cells  of  the  leaf,  but 
also  in  the  vessels  themselves.  Shreve  (40)  has  submitted  evi- 
dence, at  least  theoretical,  showing  that  variations  in  the  tran- 
spiring power  are  due  to  variations  in  the  water-holding  capacity 
of  the  internal  tissue.  Using  the  Dixon  (16,  17,  18)  conception 
of  continuous  columns,  as  well  as  the  results  of  the  experiments  of 
Renner  (34,  35,  36)  upon  transpiration,  there  is  doubtless  a greater 
tension  present  upon  the  water  columns.  If  this  is  related  to 


94 


BOTANICAL  GAZETTE 


[august 


absorption  and  incipient  drying,  an  additional  force  must  be 
present  in  order  to  cause  the  water  to  be  pulled  into  the  cells  to  a 
greater  degree  than  before.  If  this  interpretation  is  correct,  the 
older  leaves  (on  account  of  their  closer  proximity  to  the  absorption 
center)  should  show  a more  complete  revival.  This  speculation 
would  necessarily  be  based  upon  the  readings  of  the  secondary 
maxima.  The  comparative  values  become  evident,  for  la  has  a 
maximum  of  0.93,  falls  to  0.59,  and  subsequently  returns  to  0.91; 
for  Ia2  the  maximum  is  1.00  and  comes  back  from  0.97  to  0.86. 
This  fall  and  subsequent  rise  are  independent  of  the  evaporation 
rate. 

The  march  of  the  foliar  transpiring  power  is  more  or  less  definite. 
This  is  especially  true  as  it  bears  upon  maximum  and  minimum 
values.  For  I a the  maximum  value  occurs  on  the  nth  hour  with 
an  index  of  o . 95,  while  the  minimum  value  o . 50  is  on  the  23d  hour. 
The  ratio  between  maximum  and  minimum  is  1.9.  For  Ia2  the 
maximum  value,  1 .00,  is  in  evidence  on  the  10th  hour,  while  the 
reading  0.58  on  the  2d  hour  of  July  22  gives  the  minimum  value. 
The  ratio  in  the  latter  case  is  1.72. 

Another  important  feature  presented  by  the  present  series  is 
the  high  value  on  the  17th  hour  of  July  22.  In  the  previous 
experiments  which  have  dealt  with  foliar  transpiring  power,  there 
has  been  a fall  in  the  transpiring  power  value  after  the  secondary 
rise.  It  is  noticed  that  the  evaporation  during  the  afternoon  is 
rather  intense,  being  1.6.  The  high  value  of  the  transpiring  power, 
therefore,  is  without  question  due  to  the  high  evaporating  power 
of  the  air. 

In  formulating  a graph  (fig.  3)  from  the  data  presented  in  the 
march  of  foliar  transpiring  power,  the  general  feature  is  the  high 
foliar  transpiring  power  before  the  time  of  greatest  diurnal  evapo- 
ration. For  both  leaves  the  maximum  is  reached  at  the  nth  hour, 
when  the  index  is  1 . 00.  This  value  is  retained  for  I6X  until  the  12th 
hour,  and  for  I b2  until  the  13th  hour.  The  minimum  value  (0.47) 
for  1^!  occurs  on  the  2 2d  hour,  while  for  I b2  (0.44)  it  occurs  on  the 
1 8th  hour.  The  ratio  of  maximum  to  minimum  or  of  day  value 
to  night  value  is  2 . 1 for  I bz  and  2 .3  for  I b2.  The  sudden  drop  in 
the  afternoon  reading  on  the  14th  hour  is  equally  as  striking  as  that 


BAKKE— WILTING 


95 


1918] 


TABLE  IV 


Data  for  march  of  indices  of  foliar  transpiring  power  for  llelianlhus  plant 
ib  (maxima  in  bold  faced  type,  minima  in  italics) 


Leaf 

NUMBER 

Time  of 

OBSERVATION 

Time  of  color 

CHANGE  IN  SECONDS 

Index  of  transpiring  power 

Evaporation 

from 

STANDARDIZED 
ATMOMETER.CC. 
PER  HOUR 

Lower 

surface 

Upper 

surface 

Lower 

surface 

Upper 

surface 

Entire 

leaf 

July  21 

Ibz 

18:  25 

27 

63 

0.89 

0.38 

0.64 

I b2 

18:25 

44 

73 

0-55 

0-33 

O.44 

Ibz 

I9  '• 2 5 

40 

67 

0.72 

0-43 

0.58 

O.  22‘ 

Ib2 

19:30 

44 

81 

0.65 

0-35 

0.50 

I61 

20:25 

4i 

68 

0.71 

0-43 

0-57 

0.15 

Ib2 

20:25 

43 

78 

0.67 

0-37 

O.52 

Ibz 

21:15 

44 

90 

0.66 

0.32 

O.49 

O.  l6 

1^2 

21 : 20 

39 

105 

0.74 

0.28 

O.51 

Ik 

22:  20 

52 

95 

0.60 

o-33 

O.47 

0.09 

Ib2 

22:25 

50 

97 

0.62 

0.32 

O.47 

I6X 

23:15 

47 

85 

0.68 

0.38 

o-53 

0.15 

Ib2 

23 : 20 

43 

82 

0.74 

o-39 

o-57 

I^i 

24:20 

53 

78 

0.62 

0.42 

0.52 

0.18 

Ib2 

24:20 

49 

73 

0.67 

0-45 

0.56 

July  2 2 

Ibz 

i : 20 

5i 

78 

0.67 

0.44 

0.56 

O.  12 

I b2 

i : 20 

47 

7i 

O.72 

0.48 

0.60 

I&i 

2 : 20 

57 

75 

0.63 

0.48 

0.56 

015 

1^2 

2 : 20 

50 

72 

O.72 

0.50 

0.61 

Ibz 

3:20 

46 

70 

0.78 

0.52 

0.65 

0.15 

Ib2 

3:20 

4i 

70 

0.88 

0.52 

0.70 

I61 

4:20 

49 

90 

0-73 

0.40 

0-57 

0.15 

U2 

4:  20 

43 

75 

0.84 

0.48 

0.66 

Ibz 

5:20 

48 

82 

o-75 

0.44 

0.60 

0.15 

Ib2 

5:25 

40 

85 

0.90 

0.38 

0.64 

Ibz 

6: 20 

42 

50 

0.81 

0.68 

o-75 

0.15 

Ib2 

6:  20 

46 

55 

0.74 

0.62 

0.68 

I&i 

7:i5 

3i 

37 

0.74 

0.62 

0.68 

0.15 

Ib2 

7:i5 

26 

42 

0.89 

o-55 

0.72 

Ibz 

8:25 

25 

3i 

0.92 

0.74 

0.84 

0.15 

Ib2 

8:25 

■■5 

28 

0.92 

0.82 

0.87 . 

Ibz 

9: 10 

25 

30 

0.84 

0.70 

0.77 

0.22 

Ib2 

9^5 

21 

22 

1 .00 

o-95 

0.98 

Ibz 

10: 10 

25 

26 

0.84 

0.81 

0.83 

O.58 

Ib2 

10: 10 

23 

24 

0.91 

0.88 

0.90 

Ibz 

11:10 

10 

19 

1 . 00 

1 . 00 

1 . 00 

I&2 

11 : 10 

T9 

19 

1 .00 

1 .00 

1 .00 

I61 

12:10 

18 

18 

1 . 00 

1 .00 

1 .00 

I6a 

12:10 

18 

18 

1 .00 

1 .00 

1 .00 

Ibz 

13:15 

16 

16 

o-93 

o-93 

o-93 

I .O 

Ib2 

13:20 

15 

15 

1 .00 

1 .00 

1 .00 

Ibz 

14:15 

14 

i5 

0.71 

0.67 

0.69 

I . 2 

Ib2 

14:  20 

i7 

17 

o-59 

o-59 

o-59 

Ibz 

15:20 

15 

15 

°-93 

o-93 

o-93 

I . 2 

Ib2 

15:20 

16 

17 

0.88 

0.82 

0.85 

Ibz 

16:25 

16 

18 

0.94 

0.83 

0.89 

1.6 

Ib2 

16:25 

17 

19 

0.88 

0.79 

0.84 

I61 

17:10 

19 

18 

o-95 

1 .00 

0.98 

1 .0 

Ib2 

17:15 

20 

23 

0.9c 

0.78 

0.84 

Ibz 

18: 10 

27 

30 

0.74 

0.67 

0.71 

1 .0 

Ib2 

18:15 

30 

33 

0.67 

0.61 

0.64 

g6 


BOTANICAL  GAZETTE 


[august 


noted  for  series  I a.  For  I bx  the  drop  really  begins  on  the  12th  hour 
and  falls  from  1.00  to  0.69,  giving  a difference  of  0.31;  for  I b2 
the  fall  is  from  1.00  to  0.59,  giving  a difference  of  0.41.  The 
recovery  for  lbl  is  from  0.69  to  0.93,  and  for  I b2  is  from  0.59  to 
0.85.  The  difference  value  is  0.24  in  one  case  and  0.26  in  the 
other.  It  was  pointed  out  for  series  la  that  the  recovery  of  the 
older  leaf  is  more  marked  than  that  of  the  younger  leaf.  This 


Fig.  3 


feature  is  again  borne  out  in  the  present  series,  where  the  values 
for  I bx  are  in  excess  of  those  of  I b2. 

For  the  reason  that  the  leaves  of  series  lb1  are  very  nearly  of  the 
same  age,  the  same  variation  as  set  forth  in  the  previous  season 
will  not  be  in  evidence.  The  minimum  values  are  slightly  lower. 
As  a result  the  ratios  between  maxima  and  minima  are  respec- 
tively 2.19  and  2.27.  The  same  high  foliar  transpiring  power  is 
present  at  the  17th  hour.  This  agrees  with  the  former  series. 
The  data  submitted  in  table  V give  the  march  of  foliar  transpiring 
power  during  the  process  of  wilting  from  July  24- August  7. 


BAKKE— WILTING 


97 


1918] 


TABLE  V 


Indices  of  foliar  transpiring  power  during  progress  of 
wilting  of  Helianthus  plant  i a 


July 


July 


July 


July 


July 


July 


July 


July 


August 


August 


August 


August 


August 


August 


August 


me  of  observation 

Index  of  transpiring  power 
entire  leaf 

Evaporation 
from  standard- 
ized atmometer, 
cc.  per  hour 

la. 

la. 

[ 9^5 

0.51 

0.68 

I .OO 

24  -j  14:00 

0-53 

0.54 

I.23 

[20:00 

0.28 

0-39 

0-73 

[ 9:iO 

0-49 

0. 29 

O.24 

25  j 14:00 

O.27 

0.23 

O.71 

[21:00 

0-59 

0.31 

0.66 

r f 9:25 

0.51 

0.31 

0. 11 

26  s 14: 10 

O.70 

0. 20 

0.91 

[21:15 

0.49 

0.21 

0.62 

f 9:25 

0-43 

0.20 

0.02 

27 114:05 

0.65 

0. 18 

1.25 

[22:25 

0.69 

0.18 

0.51 

f 9:00 

0.38 

0. 16 

0. 12 

28114:35 

0.22 

0. 14 

1 . 12 

[21:00 

O.36 

0. 19 

0.66 

[ 9:20 

0-33 

0. 18 

0.13 

29  1 14:00 

u 

0.15 

0.80 

[21:00. 

a 

0.13 

0.69 

f 9:05 

u 

0. 21 

0.28 

30  \ 14:15 

u 

0. 12 

0. 18 

[21:00 

a 

0. 16 

0.91 

fIo:o5 

a 

0.34 

0.26 

31  114:15 

u 

0.17 

1 . 11 

[21:05 

a 

0.09 

0.66 

[10:05 . 

« 

0. 17 

0. 24 

1 j 14 : 00 

u 

0.15 

0.71 

[21:05 

u 

0.13 

0.41 

[10:10 

a 

0.14 

0.24 

2 1 14: 10 

a 

0.15 

1.09 

[22:45 

u 

0.16 

0-53 

[10:00 

a 

0.14 

0.34 

3 ji4:oo 

u 

0. 12 

1.50 

u 

0.16 

0-37 

[10:00 

a 

0.17 

0.22 

4 j 14:00 

u 

0. 12 

1.03 

[21:00 

a 

0.17 

1. 18 

[10:10 

a 

0.17 

0-33 

5 114:05 

a 

1. 17 

0.77 

[21:00 

u 

0. 11 

0. 21 

, f 9:30 

u 

0. 14 

0. 11 

6114:10 

u 

0. 11 

1.49 

[21:00 

u 

0.13 

1 . 10 

7 Jio:oo 

a 

0.13 

0.49 

'114:05 

u 

0.23 

o.74 

98 


BOTANICAL  GAZETTE 


[august 


From  the  tabulated  data  of  table  V,  and  from  graph  (fig.  4)  of 
series  I a,  it  is  noticed  that  there  is  a marked  decrease  in  foliar  tran- 
spiring power  from  July  24 
up  to  the  time  when  the 
plant  wilts.  The  transpir- 
ing power  of  leaf  I az  is 
very  irregular.  There  is 
no  doubt  but  that  the  plant 
has  attained  its  permanent 
wilting  point  on  July  29, 
but  because  the  leaves  of 
this  series  are  somewhat 
older  than  I a2,  and  as  they 
are  located  nearer  the  ab- 
sorptive center  their  action 
will  be  more  or  less  modi- 
fied by  the  presence  of  the 
younger  leaves  at  the  tip. 
^ For  the  leaf  I a2  there  is  a 
g marked  decrease  in  the 
foliar  transpiring  power 
from  July  24  to  July  30, 
the  foliar  transpiring  power 
being  especially  high  on 
August  1.  This  feature  is 
probably  in  response  to  the 
exceedingly  high  tem- 
perature at  that  time.  The 
evaporation  from  the 
standardized  atmometer 
bears  out  this  statement. 
From  August  1 to  August  7 
the  index  of  foliar  tran- 
spiring power  proceeds 
almost  in  a straight  line, 
except  for  small  dips  occur- 
ring in  the  majority  of 


BAKKE— WILTING 


99 


1918] 

cases  when  the  evaporation  was  at  its  highest.  This  part  of  the 
graph  conforms  with  the  one  obtained  when  the  plants  were  lifted 
from  the  soil.  On  August  7 the  index  of  foliar  transpiring  power 
increases  from  0.13  to  0.23,  or  77  per  cent  from  the  10th  hour  to 
the  14th  hour.  On  the  previous  day  it  dropped  from  o . 14  to  o . 1 1, 
while  on  the  preceding  day  the  two  indices  were  the  same.  At  no 
other  time  during  the  march  was  there  such  a great  percentage 
increase. 

In  obtaining  the  ratio  between  the  day  reading  and  the  night 
reading  for  24  consecutive  hours,  the  day  readings  were  usually 
made  between  the  9th  and  the  10th  hours.  For  the  night  readings 
there  was  no  need  of  selection  as  only  one  reading  was  taken. 
Beginning  with  July  24,  and  continuing  until  July  29  (time  of 
wilting),  the  transpiring  power  indices  representing  the  day  read- 
ings for  I az  are  0.51,  0.49,  0.51,  0.43,  0.38,  0.33,  while  the 
corresponding  night  values  are  0.28,  0.59,  0.49,  0.69,  0.36,  0.33. 
The  respective  ratio  values  are  1.82,  0.83,  1.04,  0.62,  1.05.  On 
July  21-22  the  ratio  between  the  reading  on  the  9th  hour  and  the 
reading  on  the  21st  hour  is  0.88:0.52,  or  1.7.  For  the  entire 
24-hour  period  the  maximum  and  minimum  ratio  is  1.9.  The 
only  normal  ratio  is  the  first.  It  is  interesting  to  note  that  for  leaf 
Iaz  the  minimum  is  normally  o . 50.  During  the  progress  of  wilting 
the  maximum  does  not  get  below  this  point  until  July  27;  after 
that  it  is  below  the  usual  minimum. 

From  July  24  to  August  7 (time  of  wilting)  the  corresponding 
indices  for  I a2  are  present;  for  the  morning  0.68,  0.29,  0.31,  0.20, 
0.16,  0.18,  0.21,  0.34,  0.17,  0.14,  0.14,  0.17,  0.17,  0.14,  0.13; 
for  the  night  0.39,  0.31,  0.21,  0.18,  0.19,  0.13,  0.16,  0.09,  0.13, 
0.16,  0.16,  0.17,  o.n,  0.13.  The  ratio  of  the  day  (morning) 
readings  to  the  night  readings  is  respectively  1.74,  0.94,  1.11, 
0.84,  1.38,  1. 31,  3.78,  1. 31,  0.88,  0.88,  1. 00,  1.54,  1.08.  For 
July  21-22  the  ratio  of  maximum  to  minimum  for  I a2  is  1 .72.  For 
the  corresponding  hours  the  ratio  is  0.95:0.66,  or  1.44.  On  this 
basis,  therefore,  the  readings  of  the  first  day  are  normal,  in  that 
the  ratio  is  approximately  the  same  as  for  the  maximum  to  the 
minimum  on  July  21-22  (1 .72).  Also  the  maximum  values  during 
the  march  of  wilting,  with  the  exception  of  the  first  reading,  are  all 


IOO 


BOTANICAL  GAZETTE 


[august 


below  the  minimum  set  during  the  daily  march  of  foliar  transpiring 
power  for  July  21-22. 

The  data  presented  in  table  VI  give  results  that  harmonize  with 
those  of  table  V.  As  was  stated  in  connection  with  the  march  of 

TABLE  VI 

Index  of  foliar  transpiring  power  during  process  of 
wilting  of  Helianthus  plant  i b. 


Time  of  observation 


Index  of  transpiring  power 
entire  leaf 


Ik 


Mi 


Evaporation 
from  standard- 
ized atmometer, 
cc.  per  hour 


July  24 
July  25 
July  26 
July  27 
July  28 
July  29 
July  30 
July  31 
August  1 

August  2 
August  3 


9:20. 

14:00. 

20:40. 

9:00. 

14:00. 

21:10. 

9:35- 

14:15- 

2 t : 00 . 
9:30. 

14:15- 

22:00. 

9:25- 

i4:45- 

21:00. 

9:30. 

14:00. 

21:00. 

9:10. 

14:20. 

21:00. 

[10: 10. 
j 14:20. 
(21:15. 

'10: 10. 
14:00. 
21 : 10. 

[10:15. 

ji4:i5- 

(22:50. 

fio: 10. 
\i4:oo. 


o.  17 
o.  18 
o.  18 

o.  12 
o.  17 
o.  19 

o.  12 
o.  18 
0-35 


0.25 
o.  24 
o.  14 

0.31 
o.  14 
o.  26 

o.  26 
o.  17 
0.13 


0.15 

O.  21 

o.  14 

0.20 
O.T4 
O.  21 

0.28 
O.  12 

o.  19 

0.13 
o.  18 

O.  20 


o.  14 
0.15 
015 

o.  14 
0.17 
o.  10 


0.15 

O.  20 


I .OO 

I.23 
0-73 
o.  24 
0.71 
0.66 

o.  11 
0.91 
0.62 

0.02 


0.13 

0.80 

0.69 

0.28 

1.88 

0.91 

0.  26 

1 . II 

0.60 

0.24 

0.71 

0.41 


foliar  transpiring  for  24  consecutive  hourly  periods,  the  leaves 
selected  here  were  closer  together,  and  considering  the  relation 


BAKKE— WILTING 


IOI 


1918] 

which  is  present  between  the  leaves  it  would  be  expected  that  the 
variation  would  not  be  as  great. 

The  data  given  in  table  VI  show  slowly  decreasing  values; 
however,  the  decrease  is  not  marked.  The  highest  foliar  transpir- 
ing power  for  I6X  is  0.35,  while  the  lowest  is  0.10.  The  highest 
point  as  here  set  forth  occurs  on  the  21st  hour  on  July  26,  while 
on  the  following  day  the  index  at  the  same  time  is  0.34.  After 
that  there  is  a slight  fall,  although  this  is  not  true  for  all  the 
readings,  for  on  July  30  the  index  is  0.31.  Even  at  the  time  of 
wilting,  the  index  at  the  morning  hour  is  0.26.  From  July  29 
to  July  31  the  maximum  values  are  approximately  the  same. 
This  is  also  true  of  the  minimum  values.  The  last  reading  for  the 
I61  series  occurs  on  July  31  and  gives  an  index  of  o.  13. 

For  leaf  I b2  the  values  are  strikingly  similar  to  those  of  the 
leaf  situated  just  below  it  upon  the  stem.  The  highest  transpir- 
ing power  index  for  the  entire  time  is  only  0.28,  and  occurs  at  9 : 35, 
July  26;  while  the  minimum  value  0.10  occurs  on  the  14th  hour 
of  July  29  and  on  the  21st  hour  of  July  31.  The  data  of  table  VI, 
represented  graphically  in  fig.  5,  show  a gradual  dropping  off  of 
the  day  maximum  values  from  July  26  to  July  30.  From  July  30 
to  August  3,  with  the  exception  of  August  1,  the  graph  of  wilting  is 
practically  a straight  line.  On  August  3 there  is  a marked  increase, 
considering  that  the  entire  period  has  given  a low  index  throughout 
(from  0.15  to  0.20,  or  an  increase  of  33.3  per  cent).  Usually 
during  the  march  of  foliar  transpiring  power  a drop  is  registered 
at  the  14th  or  15th  hour.  On  July  27  there  is  an  increase  of  the 
index  from  o . 13  to  o . 18  (38.46  per  cent)  and  on  July  31  from  o . 14 
to  o.  17  (21.43  Per  cent).  On  account  of  the  comparatively  small 
deviation  between  the  maximum  and  minimum  values  throughout, 
the  increase  in  the  transpiring  power  of  one-third  on  August  3 
becomes  more  significant  than  the  graph  shows  (fig.  5). 

The  ratio  between  the  day  indices  and  night  indices  is  presented 
as  before,  the  readings  of  the  9th  and  21st  hours  being  used.  On 
July  24  the  morning  reading  is  0.17;  on  the  following  days  the 
average  foliar  indices  of  transpiring  power  for  leaf  Ifri  are  o . 12,  o . 12, 
0.10,  0.22,  0.25,  0.31,  0.26.  The  corresponding  night  values  are 
0.18,  0.19,  0.35,  0.34,  o.  12,  0.14,  0.26,  0.13.  The  corresponding 


102 


BOTANICAL  GAZETTE 


[august 


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Fig. 


BAKKE— WILTING 


103 


1918] 

ratios  between  the  day  and  night  readings  are  respectively  0.95, 
0.63,  0.34,  1.83,  1.79,  1. 19,  2.00.  The  ratio  between  the  index 
of  foliar  transpiring  power  for  the  9th  and  21st  hours  on  July  21-22 
is  1 .57.  The  ratio  between  the  day  maximum  and  the  night  mini- 
mum at  that  time  is  2 . 1 1 . 

Taking  into  consideration  that  the  minimum  for  I^  during  the 
march  of  foliar  transpiring  power  on  July  21-22  is  0.49,  the  maxi- 
mum values  during  the  march  of  wilting  are  extremely  low  from  the 
initial  to  the  final  point  of  wilting.  On  account  of  the  drop  in 
the  maximum  and  the  constant  retentive  character  of  the  minimum, 
the  index  here  is  larger  than  recorded  previously.  The  leaf  was 
completely  wilted  on  July  31. 

The  day  values  for  I b2  taken  at  the  same  time  as  before  are 
0.15,  0.20,  0.28,  0.13,  0.16,  0.14,  0.14,  0.21,  0.17,  0.15;  while 
the  corresponding  night  values  are  0.14,  0.21,  0.19,  0.20,  0.22, 
o.n,  0.15,  0.10,  0.14,  0.12.  The  ratios  between  these  two  sets 
are,  in  order  of  their  occurrence,  1 .07,  0.95,  1.45,0.65,0.59,  1.45, 
0.93,  1.40,  1.50,  1.42.  The  ratio  between  the  readings  for  the 
9th  hour  and  the  readings  of  the  21st  hour  on  July  21-22  for  Helian- 
thus  leaf  I b2  is  0.98:0.51,  or  1.92.  The  ratio  between  the  maxi- 
mum and  the  minimum  is  2.23.  In  none  of  these  cases  can  the 
proportion  be  regarded  as  normal.  This  plant  from  the  beginning 
is  evidently  in  a partially  wilted  state. 

In  comparing  the  march  of  foliar  transpiring  power  during  the 
process  of  wilting  for  the  two  series  la  and  I b,  there  would  natu- 
rally be  some  variation.  The  wilting  of  series  la  extends  over  a 
longer  period  (to  August  7),  while  that  of  I b reaches  its  permanent 
wilting  point  on  August  3.  In  both  cases  the  older  leaf  wilts  long 
before  the  younger  leaf.  However,  leaf  la  has  a greater  range 
of  foliar  transpiring  power.  Leaf  Iax  wilts  before  Ia2;  likewise  I bx 
before  I b2. 

The  ratio  between  the  morning  and  the  night  readings  of  each 
day  gives  in  the  majority  of  cases  a value  that  is  less  than  the 
normal.  For  la  all  the  results  are  either  near  1.00  or  below  it 
except  for  the  first.  As  the  maximum  value  is  above  that  of  the 
usual  minimum  the  result  cannot  be  anything  but  normal.  With 
la  the  ratio  on  July  31  is  extremely  high,  probably  being  due  to  the 


104 


BOTANICAL  GAZETTE 


[august 


extremely  high  evaporation.  Why  there  should  be  such  a decrease 
at  the  2 ist  hour  is  not  known.  The  first  ratio  1.74  is  approxi- 
mately equal  to  the  normal.  The  day  reading  is  o .68.  The  ratio 
on  August  5 is  1 . 54,  but  the  9th  hour  reading  gives  a value  that  is 
much  lower  than  the  usual  minimum.  With  a slight  decrease 
in  the  minimum,  the  ratio  between  the  two  becomes  greater  than 
before.  From  these  data  on  the  basis  of  the  ratio  between  day  and 
night  foliar  transpiring  power  values  it  is  evident  that,  if  the  ratio 
is  to  be  used  during  the  process  of  wilting,  it  can  only  be  applicable 
when  the  maximum  is  greater  than  the  usual  minimum.  Through- 
out the  series  of  both  la  and  lb  the  ratio  does  not  deviate  very  far 
from  unity,  but  in  the  formation  of  the  ratio  there  is  an  evidently 
greater  corresponding  decrease  in  the  day  value  as  compared  with 
the  night  reading.  In  both  cases  the  extent  of  daily  fluctuation 
for  the  younger  leaves  is  very  small  after  the  first  day. 

The  rate  of  evaporation  throughout  fluctuated  considerably, 
but  is  unusually  high  for  the  climate  of  Chicago.  There  is  neverthe- 
less no  close  agreement  between  evaporation  and  foliar  transpiring 
power  during  the  march  of  wilting.  Plants  similar  to  the  ones 
used  in  the  experiment  were  treated  like  la  and  lb  and  were  placed 
in  a moist  chamber  at  their  respective  times  of  wilting.  They 
behaved  in  a similar  manner  and  failed  to  recover  in  the  allotted 
time.  Although  the  plants  were  watered  at  the  same  time  with 
approximately  the  same  amount  of  water,  figs.  5 and  6 show 
indirectly  that  there  was  much  difference  in  the  soil  moisture 
content.  The  plant  designated  as  lb  was  larger  than  la,  and  would 
be  expected  to  wilt  first.  This  observation  is  borne  out  in  the 
experiment.  It  is  also  evident  from  an  examination  of  the  two 
graphs  that  the  soil  of  16  was  drier  at  the  beginning  than  that  of 
la,  as  the  indices  of  foliar  transpiring  power  are  much  smaller. 

STOMATAL  DIFFUSION 

The  index  of  foliar  transpiring  power  in  its  very  definition  is 
associated  with  that  of  vapor  tension.  The  decrease  in  the  index 
of  foliar  transpiring  power  such  as  is  present  at  night  during 
the  daily  march  represents  a great  force.  A solution  may  also 
carry  with  it  just  as  great  a force.  Livingston  (27),  com- 


BAKKE— WILTING 


1.05 


1918] 

menting  upon  Fitting’s  (19)  work  upon  the  osmotic  pressure 
present  in  desert  plants,  makes  the  statement  that  with  the 
lowering  of  the  vapor  tension  10  per  cent  there  is  represented  a 
pressure  of  100  atmospheres.  In  an  examination  of  the  graph  in 
table  I there  is  a reduction  in  the  index  from  0.92  to  0.19  during 
the  process  of  wilting.  This  therefore  represents  an  approximate 
pressure  of  800  atmospheres.  For  plant  B there  would  be  an 
approximate  pressure  of  700  atmospheres.  Table  V and  fig.  4 
give  leaf  I ax  as  being  able  to  withstand  a pressure  of  666  atmos- 
pheres and  leaf  I a2  860  atmospheres.  Leaf  Ibz,  with  an  index  o . 10, 
suggests  a pressure  as  high  as  900  atmospheres.  At  that  time 
the  margin  of  the  leaf  was  sufficiently  dry  so  that  the  clip  could  not 
be  used  without  causing  injury.  This  status  becomes  all  the  more 
pertinent  when  it  is  compared  with  the  data  submitted  by  Shull 
(41),  where  the  force  present  in  air  dry  seed  ( Xanthium ) is  equiva- 
lent to  1000  atmospheres. 

During  -the  daily  march  of  foliar  transpiring  power  there  is 
usually  considerable  variation  (figs.  2,  3),  even  when  a plant  is 
supplied  with  an  optimum  amount  of  water.  The  sudden  rise  in 
the  foliar  transpiring  power  immediately  after  sunrise,  as  set  forth 
by  Bakke  and  Livingston,  gives  credence  to  the  view  that  the 
stomata  open  quickly  at  this  time.  In  using  the  porometer  and 
standardized  cobalt  paper  squares  simultaneously,  Tre lease 
and  Livingston  (42)  find  that  during  the  daily  march  there  is 
considerable  agreement  between  the  porometer  readings  and  the 
readings  of  the  foliar  transpiring  power  by  the  method  of  stand- 
ardized cobalt  chloride  paper.  From  the  results  obtained  in  their 
investigation  they  concluded  that  the  porometer  gives  readings 
which  show  the  extent  of  stomatal  diffusion. 

Darwin  (12,  13),  using  the  horn  hygroscope  and  the  tempera- 
ture method,  has  shown  that  during  wilting  there  is  a temporary 
opening  of  the  stomata.  Darwin  and  Pertz  (14),  using  the 
porometer,  have  demonstrated  that  a similar  condition  is  present 
during  wilting.  Laidlow  and  Knight  (26)  in  their  work  upon 
stomatal  behavior  during  wilting,  where  they  employ  a recording 
porometer,  have  confirmed  the  results  of  Darwin  and  Pertz,  in 
that  the  stomata  open  temporarily  during  wilting.  For  Phaseolus 


io6 


BOTANICAL  GAZETTE 


[august 


vulgaris  the  maximum  diffusion  occurred  about  5 minutes  after  the 
leaf  was  severed  from  the  stem,  while  in  the  case  of  the  thick  leaf  of 
Prunus  Laurocerasus  nearly  20  minutes  elapsed  before  the  maximum 
stomatal  opening  was  reached.  Kamerling  (23)  found  that  when 
Rhipsalis  cassytha  had  lost  1 to  4 per  cent  of  its  normal  water 
supply,  the  amount  of  transpiration  per  unit  time  increased,  and 
later  when  the  loss  in  weight  had  reached  a certain  point,  varying 
from  6 to  10  per  cent,  the  transpiration  again  diminished.  Kamer- 
ling is  of  the  opinion  that  the  increase  in  transpiration  is  due  to 
the  opening  of  the  stomata.  Lloyd  (31),  on  the  other  hand,  failed 
to  find  this  temporary  opening. 

The  evidences  at  hand  support  the  conclusion  that  the  stomata 
open  for  a short  time  during  wilting.  The  time  is  short,  however, 
and  there  is  no  evidence  that  the  stomata  ordinarily  open  up  during 
the  early  stages  of  wilting  and  continue  to  be  open  until  the  plant 
has  attained  its  permanent  wilting  point.  This  point  is  important 
in  connection  with  argument  presented  for  the  break  in  the  water 
columns. 

In  order  to  prove  that  the  stomata  open  only  during  the  early 
stages  of  wilting,  the  porometer  as  modified  and  used  by  Knight 
(24)  was  resorted  to.  The  plant  was  attached  to  the  aspirator  and 
allowed  to  remain  until  the  leaves  were  partially  wilted.  Tests 
were  made  upon  plants  grown  in  the  greenhouse  and  later  trans- 
ferred to  the  laboratory  and  plants  which  had  been  grown  con- 
tinuously in  the  greenhouse.-  The  plants  in  the  laboratory  were 
kept  for  5 days  before  experimentation  was  begun.  The  tests 
of  this  series  were  begun  on  November  28,  1917,  and  continued 
until  December  3,  1917;  readings  were  made  at  three  different 
times  of  the  day.  This  conforms  with  the  plan  adopted  in  making 
the  readings  of  the  foliar  transpiring  power.  Evaporation  was 
recorded  by  means  of  a standardized  cylindrical  form  of  atmom- 
eter  of  the  Livingston  type.  The  data  are  given  in  table  VII. 

In  an  examination  of  the  data  presented  in  table  VII,  readings 
were  not  taken  until  the  16th  hour  on  November  29,  3 days  after 
all  watering  had  ceased.  The  time  elapsing  between  two  suc- 
cessive bubbles,  as  ascertained  by  means  of  a stop  watch,  was  found 
to  be  160  seconds.  At  the  nth  hour  it  took  140  seconds,  while 


BAKKE— WILTING 


107 


1918] 


on  December  1 at  10:30  it  took  246  seconds.  From  that  time 
until  December  3 there  was  not  much  variation  either  at  night 
or  during  the  day. 

TABLE  VII 

POROMOTER  READINGS  DURING  PROGRESS  OF  WILTING  OF  A 

Helianlhus  annuns  plant  growing  in  laboratory 


Time  of  observation 

Rate  of  evapora- 
tion from  standard- 
ized atmometer, 
cc.  per  hour 

Rate  of  flow,  time 
interval  in  seconds 
between  succes- 
sive bubbles 

November  27 

-1 

16:00. .. . 

8:00.... 

2o< 

\I5:30. • • • 

i-45 

29 

16:00. .. . 

0.72 

160 

I 

[11:00.... 

1 . 12 

140 

30 

14:30. • • • 

0.79 

150 

1 

[20:00.... 

i-34 

146 

! 

[10:30 

0.97 

246 

December  i-j 

14:30. . . . 

1 . 11 

266 

[20:00. .. . 

0-54 

259 

'10:30 

1.79 

276 

2- 

14:30 

1. 71 

260 

20:00. . . . 

1. 41 

262 

3 

10:30 

1 . 70 

1 

269 

The  data  of  the  series  grown  continually  in  the  greenhouse, 
where  the  evaporation  was  very  low,  are  given  in  table  VIII. 

These  results,  which  were  obtained  at  regular  intervals  on  5 
successive  days  (November  29  to  December  3,  1917)^0  not  show 
any  marked  differences.  Considering  the  experimental  error 
which  would  be  present,  there  is  not  sufficient  difference  in  any  one 
case  to  indicate  that  stomatal  movement  was  present.  The  plants 
used  in  this  series  were  not  watered  for  3 days  before  the  beginning 
of  the  experiment. 

The  stomatal  diffusion  as  measured  by  the  porometer  was 
also  determined  for  plants  which  were  grown  for  the  same  period, 
but  were  not  subjected  to  any  extended  period  of  wilting.  The 
general  average  for  the  stomatal  diffusion  as  represented  by  the 
time  interval  between  successive  bubbles  of  the  air  intake  tube 
of  a porometer  was  found  to  be  105  seconds.  No  attempt  was 


io8 


BOTANICAL  GAZETTE 


[august 


made  to  ascertain  whether  or  not  the  stomata  were  partially 
closed  or  whether  there  was  an  increased  opening  after  a period  of 
2 hours,  as  has  been  shown  for  certain  plants  by  Iljin  (20).  At 
any  rate,  the  time  interval  in  the  case  of  intensely  wilted  Ilelianthus 
plants  is  much  smaller.  Even  if  the  stomata  should  be  partially 

TABLE  VIII 

POROMETER  READINGS  TAKEN  OF  A WILTING  Helianthus 
PLANT  GROWING  WHERE  EVAPORATING  POWER 
OF  AIR  IS  LOW 


Rate  of  evapora- 

Rate of  flow,  time 

Time  of  observation 

tion  from  standard- 
ized atmometer, 

interval  in  seconds 
between  succes- 

cc. per  hour 

sive  bubbles 

December  11  16:00.... 

T 2 i" 1 3 : °o  • • • • 
1 \i6:3o. . . . 

0-35 

O.23 

[10:30. . . . 

i31 14:30 

[20:00. . . . 

0-34 

0.65 

0-45 

0-37 

[10:30. . . . 

357 

i4i  14:30 

O.60 

360 

[20:00. . . . 

0.34 

352 

[10:30 

0.30 

3l8 

iSl  T4:o° 

0.36 

309 

[20:00. . . . 

0.40 

335 

[10:30 

0-43 

357 

16] 14:30 

0-43 

348 

[20:00. . . . 

0-49 

343 

[10:30. . . . 

O.40 

34i 

i7] I4:3° 

O.44 

365 

[20:00. . . . 

0-37 

343 

[10:30 

0-34 

398 

18] 14:30 

O.44 

389 

[20:30 

0-39 

360 

closed,  the  results  obtained  from  tables  VII  and  VIII  show  that 
during  the  march  of  wilting,  where  the  plant  acquires  its  perma- 
nent wilting  point,  the  stomatal  opening  does  not  enter  in  to  affect 
the  diffusion  or  transpirational  water  loss.  This  statement  is  in 
agreement  with  that  of  Darwin,  that  when  the  transpiration  is 
high  and  the  supply  water  insufficient  the  lack  of  water  is  a more 
important  factor  than  stomatal  changes.  It  would  be  extremely 
advantageous,  however,  to  have  the  stomatal  movement  question 


BAKKE— WILTING 


log 


1918] 

settled.  Regarding  the  stomatal  diffusion  as  a minor  factor  during 
intense  wilting,  the  problem  resolves  itself  to  the  point  where  the 
resistance  to  the  passage  is  considered.  From  the  data  given  in 
this  paper  and  in  a previous  publication  the  resistance  is  exceed- 
ingly great.  This  will  give  further  information,  therefore,  upon 
the  strength  of  the  evaporating  force  and  that  of  cohesion. 

Discussion 

In  comparing  the  results  obtained  during  the  summer  of  1915 
with  those  of  1916,  considerable  additional  evidence  is  set  forth 
which  substantiates  the  argument  advanced  by  Bakke  that  wilting 
occurs  at  a definite  point  and  is  readily  determined  by  the  use  of 
standardized  hygrometric  paper.  In  the  series  of  1915  the  average 
of  3 leaves  were  used  in  plant  I a and  2 leaves  for  plant  lb.  No 
effort  was  made  during  the  1915  season  to  obtain  the  difference  in 
the  time  of  wilting  for  leaves  of  different  ages.  The  difference, 
however,  was  probably  very  slight,  as  the  evaporation  was  exceed- 
ingly low.  At  no  time  during  the  entire  run  was  the  evaporation 
as  high  as  o . 7 cc.  per  hour,  and  usually  it  was  below  0.5  cc.  per 
hour.  The  temperature  of  the  greenhouse  was  seldom  over  28°  C. 

In  contrast,  the  evaporation  during  the  season  of  1916  was  high 
and  during  the  time  the  experiments  were  being  performed  was 
exceedingly  uniform.  It  may  be  added  that  during  the  progress 
of  the  experiment  no  rain  fell.  It  would  then  have  been  preferable 
to  have  run  the  experiments  outside,  but  in  the  climate  of  Chicago 
it  is  rather  difficult  to  obtain  such  a continued  period  of  clear 
weather.  The  usual  feature  will  then  be  a low  evaporation  at 
night,  a higher  one  during  the  forenoon,  and  the  maximum  at  the 
14th  hour.  The  high  evaporation  rate  on  July  3 1 is  not  explainable. 
It  may  be  well  to  remark,  however,  that  on  July  30  the  tempera- 
ture in  the  greenhouse  was  41 .2°  C.  and  at  the  first  hour  of  July 
31  it  was  270  C.,  almost  the  maximum  of  the  previous  year. 

It  is  again  brought  out  that  for  the  1915  and  1916  series  a point 
is  reached  where  the  foliar  transpiring  power  shows  very  little 
fluctuation.  In  the  cases  presented,  this  point  can  be  represented 
graphically  by  a line  that  is  almost  straight.  The  ratio  values  are 
not  far  above  unity  in  the  majority  of  cases,  and  sometimes  are 


no 


BOTANICAL  GAZETTE 


[august 


even  lower.  It  is  noted  also  that  the  time  element  of  this  period 
varies  greatly  in  the  two  seasons.  In  1915  it  is  comparatively 
short,  while  for  both  series  in  1916  it  is  extended  over  a consider- 
able period.  It  has  been  proved  by  the  work  of  Shreve  (34)  that 
plants  grown  under  different  environment  not  only  have  different 
anatomical  characters  but  also  have  a different  rate  of  transpira- 
tion. 

On  the  basis  therefore  of  a possible  change  as  a result  of 
environment,  it  can  safely  be  asserted  that  this  is  the  reason  for 
the  short  span  in  1915  and  the  long  one  in  1916.  Why  or  how 
the  plant  establishes  such  an  apparent  equilibrium  cannot  be 
stated.  This  equilibrium  represents  the  greatest  force  or  tension 
which  can  be  applied  before  a plant  assumes  the  condition  of 
permanent  wilting.  A plant  such  as  A triplex  will  necessarily  have 
an  extended  period  when  this  equilibrium  is  maintained.  The 
exact  wilting  will  be  when  there  is  a serious  rupture  in  the  water 
columns. 

If  this  interpretation  is  correct,  the  1915  and  1916  series  should 
exhibit  a difference  in  the  foliar  transpiring  power  values  during 
the  so-called  equilibrium  stage.  It  would  be  expected  that  the 
1915  series  would  have  a higher  minimum  than  the  1916  series. 
This  is  evident,  for  in  1915  the  lowest  point  reached  at  any  time 
is  never  below  0.15,  while  for  the  1916  series  it  is  as  low  as  0.09  in 
one  case  and  0.10  in  the  other.  There  would  thus  seem  to  be  a 
direct  relation  between  the  time  of  the  equilibrium,  the  lowest  point 
in  the  index  of  foliar  transpiring  power,  and  the  evaporating  power 
of  the  environment.  The  point  at  which  wilting  occurs  is  defi- 
nitely marked  out.  This  point  appears  graphically  to  better 
advantage  for  the  plants  of  1915  than  for  those  of  1916;  but  the 
plants  of  1915  were  larger  and  were  grown  in  smaller  containers 
than  those  of  the  following  year.  For  the  series  of  1915  the 
permanent  wilting  occurs  on  August  21,  while  for  series  la  (1916) 
the  wilting  occurs  on  August  7,  and  for  series  lb  of  the  same  year 
the  wilting  occurs  on  August  4. 

In  this  study  the  same  conception  of  wilting  is  advanced  as 
before.  The  present  study  is  really  more  or  less  of  an  elaboration 
of  the  former.  It  is  assumed  here  that  Dixon’s  (16,  17,  18)  con- 


BAKKE— WILTING 


ill 


1918] 


ception  of  continuous  water  columns  is  in  force.  When  the  force 
of  evaporation  becomes  sufficient  to  cause  a serious  rupture  of 
these  water  columns,  then  the  plant  wilts.  Just  to  what  extent  a 
serious  rupture  can  be  regarded  cannot  be  stated,  but  it  must  be 
greater  than  the  force  of  cohesion  which  holds  the  water  particles 
together. 

The  extent  of  this  cohesion  force  has  been  sufficiently  presented 
and  advanced  by  Dixon  (16,  17,  18),  Renner  (34,  35,  36),  Ur- 
sprung  (43,  44,  45,  46,  47),  and  others  (21,  32),  and  although  the 
conclusions  have  been  criticized  by  Jost  (22),  nevertheless  they 
are  substantiated.  It  is  not  the  province  of  this  article  to  enter 
into  a critical  discussion  of  these  various  papers.  The  approxi- 
mate point  of  permanent  wilting  is  readily  ascertained  from  the 
beginning  by  taking  a series  of  readings  of  the  foliar  transpiring 
power  of  the  plant  in  question.  Care  should  be  taken  to  obtain  in 
the  series  the  maximum  and  minimum.  Although  there  is  not  any 
hard  and  fast  relation  between  the  maximum  and  the  minimum, 
when  the  moisture  in  a soil  has  been  reduced  to  the  point  where  the 
maximum  is  below  the  normal  minimum,  at  a time  of  the  normal 
maximum,  then  the  water  content  of  that  soil  has  attained  what 
the  writer  designates  as  the  critical  content.  From  this  point 
it  is  simply  a question  of  time  when  the  columns  break.  This  then 
becomes  a relatively  simple  matter. 

The  readings  giving  the  indices  of  foliar  transpiring  power 
taken  at  hourly  intervals  present  a graph  that  is  similar  to  graphs 
set  forth  previously.  The  maximum  occurs  at  a time  previous  to 
the  highest  evaporation ; the  minimum  generally  occurs  somewhere 
between  the  18th  hour  and  the  24th  hour.  There  is  a decided 
drop  in  the  afternoon,  which  occurs  at  a time  of  day  when  evapora- 
tion is  at  its  height  or  nearly  so.  There  is  a recovery  that  is  also 
conspicuous.  The  cause  for  this  resistance  has  been  advanced  by 
Shreve  (40)  as  being  due  to  the  imbibitional  forces  of  the  cell  wall 
and  of  the  colloids  of  the  protoplasm.  Although  this  feature  has 
been  noticed  wherever  the  march  of  foliar  transpiring  power  has 
been  obtained,  no  one  as  yet  has  set  forth  any  evidence  as  to  the 
length  of  time  necessary  for  recovery  to  take  place.  It  is  appar- 
ent that  the  recovery  has  been  complete  before  the  time  of  the 


1 1 2 


BOTANICAL  GAZETTE 


[august 


beginning  of  the  next  reading,  which  in  this  case  is  the  next  hour. 
A record  of  the  foliar  transpiring  power  at  hourly  intervals  at 
Chicago  gives  results  that  are  similar  to  those  obtained  for  plants 
of  the  same  species  in  southern  Arizona. 

The  series  of  1916  show  conclusively  that  the  older  leaves  are 
the  first  to  wilt.  In  an  examination  of  series  I a the  older  set  of 
leaves  is  almost  completely  dry  at  the  time  of  the  permanent  wilting 
of  the  plant.  On  July  29  the  edges  of  the  leaf  are  dry,  but  at  the 
same  time  there  is  a different  form  of  response  in  the  younger  leaves, 
in  that  the  apparent  recovery  occasioned  at  the  time  of  permanent 
wilting  does  not  present  itself.  The  same  situation  is  true  for  the 
series  I b,  where  the  older  leaves  wilt  on  July  31.  That  the  older 
leaves  are  the  first  to  wilt  has  previously  been  determined  by  a 
number  of  investigations  (15,  33).  Bakke  and  Livingston 
have  presented  evidence  that  there  is  considerable  variation  in  the 
index  of  foliar  transpiring  power  of  young  and  old  leaves.  The 
fact  that  the  younger  leaves  wilt  later  than  the  older  leaves  is  not 
necessarily  connected  with  the  environment.  This  is  true  whether 
the  evaporation  is  low  or  whether  it  is  high.  The  production  of 
the  absciss-layer  may  at  least  be  indirectly  formed  as  a result. 
Pringshein  (33)  previously  has  shown  that  young  leaves  retain 
their  freshness  for  a longer  time  than  older  ones.  This  he  ascribes 
to  a greater  osmotic  pressure.  During  the  march  of  wilting  it  is 
also  noticed  that  the  foliar  transpiring  power  index  of  the  older 
leaves  is  always  higher,  at  least  than  that  of  the  leaves  of  the 
tip.  The  older  leaves  then  give  a higher  foliar  transpiring  power 
throughout. 

There  is  also  in  evidence  during  the  march  of  wilting  not  only  a 
low  index  of  foliar  transpiring  power,  but  also  a gradual  increase  of 
the  force  in  opposition  to  the  passage  of  water.  When  for  a short 
time  there  is  an  evident  break  or  a serious  rupture,  there  is  a 
decrease  in  the  resistance,  but  an  equilibrium  with  the  atmosphere 
is  soon  reached.  The  assumption  that  there  is  a temporary  open- 
ing of  the  stomata  may  be  made  at  this  point.  Employing  the 
porometer  upon  Helianthus  plants  placed  in  an  environment  of 
high  evaporating  power  and  one  of  low  evaporating  power,  the 
author  failed  to  find  that  the  stomata  are  concerned. 


BAKKE— WILTING 


1918] 


113 


Summary 

1.  The  transpiring  power  of  plants  as  determined  by  standard- 
ized hygrometric  paper  gives  an  accurate  knowledge  of  the  internal 
water  relations  of  a plant.  The  exact  wilting  point  as  determined 
by  this  method  occurs  when  there  is  a serious  rupture  in  the  water 
columns. 

2.  During  the  daily  march  of  foliar  transpiring  power  obtained 
by  making  consecutive  hourly  readings  for  24  hours,  the  maximum 
is  attained  at  a time  previous  to  the  greatest  evaporation.  During 
the  time  of  approximate  maximum  evaporation  there  is  a marked 
fall  in  the  foliar  transpiring  power  index,  followed  shortly  by  a rise. 
The  ratio  between  the  maximum  and  the  minimum  is  more  or  less 
definite,  but  not  sufficiently  so  for  the  formation  of  any  law.  When 
the  ratio  is  reduced  to  the  point  where  it  is  in  the  neighborhood 
of  unity,  the  plant  is  in  a state  of  intense  incipient  drying.  When 
the  maximum  value  does  not  exceed  the  usual  minimum,  the  plant 
is  in  a soil  environment  which  is  critical  from  the  point  of  water 
supply,  or  almost  at  its  wilting  coefficient.  It  is  then  merely  a 
question  of  time  before  the  plant  wilts. 

3.  Evaporation  plays  an  important  part  in  the  experiment 
upon  transpiration.  A high  evaporation  gives  an  increased  tran- 
spiring power  value,  but  during  the  process  of  wilting  the  index 
of  foliar  transpiring  power  comes  to  be  independent  of  evaporation. 

4.  During  the  process  of  the  march  of  wilting  an  equilibrium 
point  is  reached  where  the  indices  of  foliar  transpiring  power  do 
not  show  much  variation.  It  is  suggested  that  the  duration  of  the 
equilibrium  gives  a measure  of  the  comparative  drought  resistance 
of  different  plants.  Helianthus  grown  in  1915  during  a rainy 
season  is  different  from  Helianthus  grown  during  1916,  when  the 
season  was  unusually  dry.  The  equilibrium  period  of  1915  was 
much  shorter  than  for  1916. 

5.  There  is  a decided  difference  in  the  time  at  which  permanent 
wilting  occurs  in  old  and  young  leaves.  The  older  leaves  will 
wilt  long  before  the  younger  ones.  The  time  interval  varies 
according  to  age. 

6.  Stomatal  movements  or  changes  are  not  important  factors 
when  the  plant  is  in  an  intense  state  of  wilting. 


BOTANICAL  GAZETTE 


[august 


114 


I wish  to  express  my  thanks  to  Dr.  H.  C.  Cowles  and  Dr.  G.  D. 
Fuller,  University  of  Chicago,  who  have  aided  me  with  suggestions 
and  criticisms. 

Iowa  State  College 
Ames,  Iowa 


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“5 


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n6 


BOTANICAL  GAZETTE 


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